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The remnants of a lost water world of rocks and oceans have been discovered by astronomers - who said they had found the basic ingredients for a habitable planet beyond our solar system.

Astronomers believe the tiny blip on their telescope screens 150 light years from Earth was once a rocky planet with huge amounts of water - the first time they have found rocks and water together in one place outside the solar system.

Water and a rocky surface are assumed to be vital for the origin of life and scientists said the discovery of both substances in deep space indicated that the basic building blocks for habitable planets might be widely distributed throughout the cosmos.

The rocky planetary body or asteroid is orbiting a star called GD 61, a "white dwarf" where the star's nuclear fuel has been exhausted. The asteroid is believed to be the remnants of a small, watery planet that was knocked out of its original orbit and pulled so close to its sun that it was shredded in the process.

Scientists said that the original planet was at least 90km in diameter, making it a minor planet in terms of size, and that it was once composed of 26 per cent water - by contrast the Earth is just 0.023 per cent water.

"At this stage in its existence, all that remains of this rocky body is simply dust and debris that has been pulled into the orbit of its dying parent star," said Professor Boris Gansicke of the University of Warwick in England. The study, published in the journal Science, used observations from the Hubble Space Telescope and the giant Keck telescope at Mauna Kea in Hawaii.